In the conventional Oldham type coupling, an intermediate coupling member is provided with relatively transverse keys that slidably engage keyways on the end coupling members to drivingly interconnect the end coupling members. The conventional Oldham type coupling, however, is not adapted for transmitting end thrust between the end coupling members, and, moreover, it is subject to relatively high sliding friction between the keys and keyways. Some coupling members such as shown in U.S. Pat. Nos. 481,780; 1,178,529; 1,411,468 and No. 2,491,364 utilize pins and sleeves for the keys and keyways of the Oldham type coupling and are capable of transmitting some end thrust through the coupling. However, the end thrust is transmitted through the same pins that also transmit the rotary torque between the coupling members and, further, the pin and sleeve type connection is still objectionable because of the relatively high sliding friction. It has also been proposed, for example as shown in U.S. Pat. No. 1,380,330 and No. 3,606,768, to provide rollers in an Oldham type coupling rotatable about an axes paralleling the coupling axes to transmit torque between the coupling members. However, such couplings were not adapted for transmitting end thrust through the coupling members.
In progressive cavity type pumps, the rotor has an eccentric motion relative to the stator and to the drive shaft and, moreover, is subjected to end thrust due to the difference in fluid pressure at the inlet and outlet ends of the rotor. In order to drivingly interconnect the shaft and rotor and to transmit end thrust therebetween, it is common practice to utilize a cardan shaft which is connected by a first universal joint at one end to the drive shaft and by a second universal joint at its other end to the rotor. The intermediate cardan shaft should be relatively long to minimize the angle to which the shaft oscillates during each revolution and the cardan type coupling thus occupies a substantial space and is, moreover, difficult to seal from the fluid being pumped.
It has been proposed as shown in U.S. Pat. No. 2,545,604 to utilize an Oldham type coupling to interconnect the rotor and shaft in a progressive cavity type pump. However, the Oldham type coupling utilized in the above patent was not capable of transmitting end thrust between the coupling members and the patent discloses an arrangement at one end of the rotor for transmitting end thrust in one direction directed to the housing and different arrangements at the other end of the rotor for transmitting end thrust on the rotor in the other direction either to the housing or directly to the input shaft.